Method for surface hardening steel and cemented carbides

ABSTRACT

A method of surface hardening materials, especially steel and cemented carbides, wherein the material to be treated while extensively excluding oxygen is brought into contact with a mixture containing approximately 5 - 70 parts by weight borium carbide, 1 - 60 parts by weight anhydrous aluminum oxide, trace amounts up to 6 parts by weight NaBF4 and trace amounts up to 10 parts by weight sodium floride. According to the invention ammonia gas is introduced during treatment temperatures exceeding approximately 400* C.

United States Patent 1191 Bopp, deceased I METHOD FOR SURFACE HARDENING STEEL AND CEMENTED CARBIDES Anton Bopp, deceased, late of Dorfstrasse 198, Meilen,

1 Switzerland by Martha Bopp, sole heir Inventor:

Filed: Nov. 16, 1971 Appl. No: 199,310 1 Foreign Application Priority Data Nov. 20, 1970 Switzerland 17283/70 US. Cl. 148/6, 148/635 Int. Cl...; C23c 11/16 Field of Search 148/63, 6.35, 16.6, 6;

117/106 C, 106 R, 107.2 R, 107.2 P, 107

References Cited UNITED STATES PATENTS 9/1958 Low l48/l6.6

ll/l971 Pike 1 ll7/107.2P 4/1962 Samuel et al. 148/63 Kunst....

[ 51 Apr. 9, 1974 3,090,702 5/1963 Commanday @1211. 117/1072 P 3,286,684 1l/1966 Aves 117/1072 P FOREIGN PATENTS OR APPLICATIONS 743,643 1/1956 Great Britain 148/63 Primary Examiner-Alfred L. Leavitt Assistant E.\'aminerCa1eb Weston Attorney, Agent, or FirmMolinare, Allegretti, Newitt and Witcoff [57 ABSTRACT 2 Claims, No Drawings METHOD FOR SURFACE HARDENING STEEL AND CEMENTED CARBIDES BACKGROUND OF THE INVENTION there is taught a technique wherein the material undergoing treatment, at a temperature above approximately hard metals or cemented carbides. The steels can be treated while in a'finished shaped condition without distortion and without embrittlement, the external appearance remaining practically unchanged.

The employed temperature can be of course increased until reaching the stability limit or boundary, depending upon the type of steel which is to be hardened. In this connection temperatures up to 620 C and l,l50 C respectively can be utilized. Further, the inventive treatment can be carried out with or without the-addition of carbon monoxide or carbon dioxide additives.

- Regarding the mixture with which the material to be treated is brought into contact such has been disclosed in my previously mentioned, copending United States application, Ser, No. 165,948, and the considerations explained therein are'also here applicable. Although 570 C, and while extensively excluding oxygen,,is

brought into contact with a mixture containing approximately 5 70 parts by weight borium carbide, 1 60 parts by weights anhydrous aluminum oxide, trace amounts up to 6 parts by weight NaBF and trace amounts up to 10 parts by weight sodium floride.

The present invention improves upon the surface hardening technique disclosed in my aforementioned copending United States application. More specifically,

it has been found that the treatment temperatures can be further reduced if during the thermal treatment there is simultaneously introduced ammonia.

The hardening of steels by treatment in an ammonia gas stream is known in the art, however the previously known procedures are only applicable to particular alloy or special steels capable of undergoing nitration. However, hardness values exceeding 900 are generally never reached, usually only 600 800 HV (Vickers hardness test), which are considerably below those attainable when practicing the teachings of the present invention.

During a roughly carried out comparative test a steel sample was gas-nitrided according to conventional techniques for hours, whereas'whe n working with the treatment can be carried 'out as described in my previously mentioned application, it is considerably more advantageous in this instance if the material to be hardened is arranged in a gas chamber, that is to say, there is utilized for instance a conventional gas nitrid-' ing furnace while employing standard operating conditions and there is introduced into such without contact with the steel the necessary hardening powder. Therequirements per square meter of steel surface for obtaining a 100 micron infusion layer or film is in the order of 3.5 to 15 grams powder. With multiple use of the powder the process uses an excess amount thereof and simultaneously there is prevented deficiencies in the infusion hard material or layer by flushing with ammothe inventive method in approximately one-tenththe' time there was obtained a considerably greater infusion depth and more than twice the hardness.

High temperature hardeners, such as for instance high-speed'steels' require for an additional boundary or rim hardening suitable carrier structures, such as for instance austenitic core structures. Suitable boundary hardening techniques must therefore be sufficiently effective at relatively low temperatures in the order of approximately 580 C 620 C in order to be ableto carry out tempering treatments without core hardness damage. I The more recent techniques for steel boriding are to a certain extent superior to the known techniques for nitride hardening, yet requireoperatingtemperatures between approximately 700 C and 900 C.

Therefore, it should be recognized that aconsiderable advance in the art is realized if it is possible to nia and b'y resorption in the furnace.

The inventive method brings about a multiple time reduction in the furnace throughput. There is also simultaneously brought about a pronounced increase in the infusion depth. The resulting hard film or coating A preferred mixture possesses for instance the j following composition:

B C 50 parts by weight Aluminum oxide. 43 parts by weight Carbon black- ,2 parts by weight NaBF 2 parts by weight NaF 3 parts by weight EXEMPYLAIRY EMBODIMENT .01 THE INVENTION;

A COMO-310-high speed steel with a performance hardness of 950 HV is heat treated for 5 minutes at a temperature in a range between 400 C and 440 C with the above composition of hardening agent while introducing ammonia gas. Even with this intentionally brief selected treatment-time the external steelhardness climbed to a value exceeding 3,700 HV. The infusion depth after 5 minutes was 500 microns. After 10 minutes there was' obtained approximately 1,000 microns infusion depth. t

From the above it will be recognized that the inventive technique, both with regard to the hardening effect as well as also with regard to the employed operating temperature, is considerably superior to the heretofore known prior art techniques. Furthermore, the inventive What is claimed is:

l. A method of surface hardening materials by a boriding and nitriding process, especially steel and cemented carbides, wherein the material to be treated while extensively excluding oxygen is brought into contact with a mixture containing approximately 5-70 parts by weight borium carbide, 1-60 parts by weight anhydrous aluminum oxide, trace amounts up to 6 parts by Weight NaBF and trace amounts up to 10 parts by weight sodium floride, the improvement com-- prising introducing ammonia gas during a treatment temperature in a range exceeding approximately 400 C up to approximately 620 C. i

2. The methodas defined in claim 1, including the step of furtherintroducing an additive selected from carbon monoxide and carbon dioxide and mixtures thereof. 

2. The method as defined in claim 1, including the step of further introducing an additive selected from carbon monoxide and carbon dioxide and mixtures thereof. 